1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine and, more particularly, to an air-fuel ratio control apparatus which switches the air-fuel ratio control point.
2. Related Art
As a conventional technology of this type, a control apparatus is disclosed in Japanese Patent Laid-open No. Sho 63-223347 wherein, with an air-fuel ratio sensor heated to a semi-activated state, feedback control of the air-fuel ratio is implemented by detecting an electromotive force output by the sensor in accordance with a Z-characteristics (stepwise output change) of the sensor. In this case, feedback control is controlled to the stoichiometric air-fuel ratio used as a target. In addition, with the air-fuel ratio sensor heated to a complete activated state, feedback control of the air-fuel ratio is implemented by detecting a limit current output by the sensor in accordance with linear characteristics of the sensor.
In recent years, however, there is a tendency that the exhaust gas regulations are in a process of being enforced, giving rise to problems that, with the substantial known technology for carrying out control based on the stoichiometric air-fuel ratio in the semi-activated state, a satisfactory effect of sufficient emission reduction and maintenance of good drivability can not be obtained. In particular, the problems are observed strikingly right after a cold-start of the internal combustion engine, a start of the engine at a low temperature.
In addition, even with the air-fuel ratio sensor heated to the semi-activated state, there has been recently raised a demand for control of the air-fuel ratio to a value located on a lean side (or a rich side) to a certain degree in close proximity to the stoichiometric air-fuel ratio point in an attempt to reduce the emission and maintain the good drivability described above. However, the existing air-fuel ratio control apparatus does not respond satisfactorily to such a demand, giving rise to a problem of the necessity to normally wait until the air-fuel ratio sensor is heated to the complete activated state.
In the case of an air-fuel ratio sensor that has only the Z-characteristics changing its output stepwisely at the stoichiometric air-fuel ratio point such as an oxygen sensor, the amount of injected fuel is controlled to a value on the rich side during heavy load driving at a high speed or the amount of injected fuel is likewise controlled to a value on the rich side in order to prevent the catalyst from being overheated. In this case, the feedback control of the air-fuel ratio can not be continued. Therefore, an open-loop control is enforced temporarily, leading to a variety of problems such as noxious exhaust emission.
It should be noted that, as a method of controlling the air-fuel ratio by using an air-fuel ratio sensor of the limit current type, that is, an air-fuel ratio sensor which generates a limit current, a method for controlling the operation of an internal combustion engine in a lean zone aimed at reduction of the fuel consumption and reduction of the amount of NOx (nitrogen oxides) instead of control based to the stoichiometric air-fuel ratio (air excess ratio .lambda.=1) has also been disclosed in Japanese Patent Laid-open No. Sho 58-158553.